For electrically driven optical devices such as optical modulator chips, it is desirable to minimize the power consumption. Reduction in the lengths of electrical connections and tracks from the driver to the optical modulator will reduce signal loss and lead to lower power consumption. In order to minimize power consumption, the lengths of electrical connections and tracks must be minimised throughout a device.
Optical modulators at high frequencies, such as microwave frequencies, typically have traveling wave transmission lines which have an impedance of 50 ohms. The transmission lines need 50-ohm termination resistors to operate effectively for impedance matching and this causes excessive power consumption. In order to minimize power consumption in this case, the optical modulators should be lumped elements without transmission lines. Furthermore, the lengths of electrical connections and tracks must be minimized.
Optical waveguide devices made in chip form are small and can be fast in operation. They can also be fabricated in volume at low cost. They are becoming increasing complex and multifunctional, and it is known to combine a number of features on a single photonic chip (for example on an SOI chip). The dimensions of such photonic chips (for example on the so-called 3 micron silicon platform) are large relative to the electronic chips that may need to be incorporated, and the resulting electrical track lengths can cause unacceptable losses.
In some complex planar photonic devices it becomes necessary either to create waveguide crossings where optical paths intersect, typically at 90 degrees. An alternative solution is to direct the light into another (parallel) plane. If light is directed from the chip, for example using a mirror to a photodiode on an adjacent chip, there are inevitable insertion losses and certain on chip features such as waveguide crossings give rise to high losses and of low yield. Also, the introduction of waveguide bends to allow effective low loss crossings can consume chip real estate. Where photonic chips include multiple detector remodulators comprising a plurality of detectors and a plurality of modulators, the presence of an input waveguide to each detector and each modulator leads to undesirable waveguide crossings. Examples are shown in FIGS. 1-3.
This invention seeks to overcome these problems and to enable faster and more compact devices with lower power consumption.
Electrical connections can be reduced by placing the electronic chip, such as a driver chip for a modulator, directly above the optical chip. Such a device is disclosed in U.S. Pat. No. 7,522,783.
Flip chipping is a technique for mounting electronic chips and has been used in the fabrication of optoelectronic devices. U.S. Pat. No. 7,978,030 discloses a high speed optical transponder in which the signal plane of a high speed circuit is flipped through non-coplanar interconnects with respect to a signal plane in a coplanar optical package; U.S. Pat. No. 6,614,949 discloses arrays of VCSELs flip-chipped to an ASIC in an optical communications interface; U.S. Pat. No. 8,373,259 discloses an optical alignment method for use when flipping an optical waveguide device onto a substrate; U.S. Pat. No. 8,798,409 discloses a flip-chip structure involving a laser chip in an optical transmitter; and US201210207426 shows a flip chip arrangement between an opto chip and an IC.